This invention relates to processes for producing hydrated iron oxide and ferromagnetic iron oxide. More specifically, the invention relates to a process for producing a hydrated iron oxide having a very narrow particle size distribution and a high aspect ratio and which is useful as a raw material for pigments, paints, catalysts and ferrites, and as a raw material for magnetic iron oxide for use as a magnetic recording medium. The invention also relates to a process by which an improved ferromagnetic iron oxide for use in coated magnetic recording media and so forth can be produced from the hydrated iron oxide.
As magnetic recording and reproducing apparatus become smaller and lighter in weight and as more information need be processed in magnetic recording, there are growing demands for increasing the density of recording in magnetic recording media, enhancing their output and rendering them less noisy. To meet these requirements, there have been more rigorous demands for better characteristics such as higher coercivity, greater amount of saturation magnetization, smaller particle size and higher degrees of orientation and packing.
To obtain a magnetic powder compliant with the above-mentioned requirements, it is important to improve the characteristics of the precursor hydrated iron oxide, such as its particle size, size distribution and aspect ratio (major-to-minor axis ratio). Under the circumstances, the conditions for the manufacture of hydrated iron oxide have been altered in various ways and subsequently put to trial use.
For example, Japanese Patent Publication No. 38000/1977 discloses a process for producing a hydrated iron oxide using a reactor comprising an air bubble column and Japanese Patent Public Disclosure No. 22638/1981 discloses a method using seed crystal prepared at room temperature. However, the first approach yields only goethite having a large particle size and the second approach yields magnetite unless strict temperature control is performed. Thus, it has been difficult to produce high-density magnetic materials from the hydrated iron oxides prepared by these methods.